Conventional fluid reservoirs for paint spray guns usually feature a cup-shaped receptacle, the underside of which is provided with an opening and a connecting part for detachably connecting the receptacle to the upper side of the paint spray gun. The connecting part is usually realized in the form of a connection piece that is arranged on the bottom of the receptacle and features an external thread that is screwed into a corresponding threaded opening on the upper side of the paint spray gun. In conventional fluid reservoirs, the connecting part is normally arranged rigidly on the receptacle. However, this results in the receptacle assuming a horizontal or nearly horizontal position when it is attached to the paint spray gun and the paint spray gun is held in certain inclined positions. In this case, the feed of the paint to the paint spray gun may no longer be ensured under certain circumstances.